1. Field of the Invention
The present invention relates to a method and apparatus for reading radiation image information. More specifically, the present invention relates to a radiation image information reading apparatus for reading radiation image information recorded on a stimulable phosphor sheet by using a plurality of line sensors.
2. Description of the Related Art
Radiation image recording and reproducing systems using stimulable phosphor (see Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 55 (1980)-116340, and 56 (1981)-104645, for example) have been in wide use. The stimulable phosphor stores a portion of energy of radiation irradiated thereon and emits light in accordance with the radiation energy stored therein upon exposure to an excitation light such as visible light and a laser beam. In such a system, radiation image information of a subject such as a human body is recorded on a stimulable phosphor sheet comprising a stimulable phosphor layer over a base material. By irradiating the excitation light such as a laser beam on each of the pixels on the sheet, light is emitted from the pixels. The light is photoelectrically read by photoelectric reading means to obtain an image signal, and an erasing light is irradiated on the sheet after the reading to release the radiation energy still remaining in the sheet.
Image processing such as tone processing and frequency processing appropriate for observation is carried out on the image signal obtained in the above manner, and the image signal after the processing is recorded on a film as a visible diagnostic image (a final image) or displayed on a high-definition CRT for diagnosis. By irradiating the erasing light on the sheet to release the residual energy, the sheet can be used repeatedly for recording radiation image information.
In a radiation image information reading apparatus used in the radiation image recording and reproducing system described above, a linear light source is used for linearly irradiating the excitation light on the sheet, in order to reduce time for reading the light, to downsize the apparatus, and to reduce cost (see Japanese Unexamined Patent Publication Nos. 60(1985)-111568, 60(1985)-236354, and 1(1989)-101540, for example). As the photoelectric reading means, a line sensor having a plurality of photoelectric conversion devices arranged in a lengthwise direction of an area in the sheet on which the excitation light is irradiated from the linear light source (hereinafter called a main scan direction) is used. The reading apparatus also comprises scanning means for moving either a combination of the linear light source and the line sensor or the stimulable phosphor sheet relative to the other in a direction substantially perpendicular to the main scan direction (hereinafter called a vertical scan direction).
Each of the photoelectric conversion devices such as a CCD sensor and a MOS image sensor comprising the line sensor has a limit called a saturation charge in terms of an amount of an electric charge to be stored therein. It is difficult to produce the sensor with a large surface area, and the sensor cannot detect light having an amount larger than a predetermined amount. As a result, an accurate measurement result is not obtained for a range in which an amount of the radiation information is large (a high dose range) in the sheet, and a range of the amount of the light readable by the sensor (a dynamic range) is narrow.
Therefore, a width of each of the photoelectric conversion devices in the vertical scan direction has been conventionally set several times smaller than a width of one pixel in the vertical scan direction in a final image. Data from several consecutive photoelectric conversion devices are read sequentially in the vertical scan direction and added to obtain data for one pixel in the final image. In this manner, the dynamic range of the line sensor can be widened.
However, in the above method, in order to add the data in the vertical scan direction, it is necessary to store, at least for each reading, all outputs from all the photoelectric conversion devices arranged in the main scan direction. Since the line sensor has the photoelectric conversion devices arranged in the main scan direction, a large-scale storage device (a line memory) is necessary to store all the outputs from each of the photoelectric conversion devices, which increases the cost.
Therefore, a radiation image information reading apparatus for solving this problem has been proposed (see Japanese Patent Application No. 2000-178327 filed by the assignee). In this apparatus, outputs from several (n) photoelectric conversion devices arranged consecutively in a lengthwise direction (the main scan direction) of a line sensor are added to comprise one pixel in a final image (this method is called pixel density conversion), instead of addition in the vertical scan direction. Therefore, each of the photoelectric conversion devices detects data whose amount is 1/nth of the amount of data for one pixel. In this manner, each of the photoelectric conversion devices can avoid reaching a saturation charge. By using such an apparatus, data can be read accurately in a range having high radiation energy stored on the sheet without using a large memory space, and a high-quality image can be obtained.
However, the length required for a line sensor used in the above radiation image information reading apparatus is approximately 35-43 cm, which is equivalent to the length of the stimulable phosphor sheet. However, due to a limitation in production, the length of the line sensor is several tens to 100 mm. Therefore, a plurality of line sensors are arranged in the direction of main scan to carry out reading. Since each of the line sensors are packaged, insensitive areas where the light is not received appear at joints of neighboring line sensors. Light emitted from an area in the sheet where the excitation light is irradiated is not detected in areas corresponding to the insensitive areas, and an artifact (false image) is created.
Therefore, a radiation image information reading apparatus for solving the above problem has been proposed (Japanese Patent Application No. 2000-217516 filed by the assignee). In this apparatus, a plurality of line sensors are arranged in the main scan direction and in the vertical scan direction, and the light emitted from the sheet is received by the photoelectric conversion devices of at least one of the line sensors. In this manner, the light can be received thoroughly.
Although an initial image comprising data each having a pixel size smaller than a pixel size of a final image is obtained by using the above apparatus, processing for converting the data is also necessary. In the processing, outputs from several lines of the line sensors in the vertical scan direction are converted into data for one line in the vertical scan direction, and pixel density conversion processing in accordance with the size of the final image is also carried out. When the processing for conversion into the data for one line is carried out on the outputs from the line sensors after the pixel density conversion processing is carried out thereon for each line in the vertical scan direction, the final image tends to become uneven. Therefore, a high quality final image cannot be obtained.
The present invention has been conceived based on consideration of the above problems. An object of the present invention is therefore to provide an apparatus enabling generation of a high quality image by preventing the image from becoming uneven. The apparatus has a plurality of line sensors arranged in a main scan direction and a vertical scan direction. The apparatus obtains an initial image comprising data having a pixel size smaller than a pixel size of a final image, and obtains the final image having the larger pixel size by processing the initial image data.
A radiation image information reading apparatus of the present invention comprises:
a linear light source for linearly emitting an excitation light to an area on a front side of a stimulating phosphor sheet having radiation image recorded therein;
detection means comprising a plurality of line sensors;
scanning means for relatively moving either a combination of the linear light source and the detection means or the sheet to the other;
reading means for obtaining initial image data; and
integration processing means for carrying out a first conversion process and a second conversion process. Each of the line sensors of the detection means comprises a plurality of photoelectric conversion devices arranged in a lengthwise direction of the area in the sheet where the excitation light is irradiated linearly (hereinafter called the irradiation area) for carrying out photoelectric conversion by receiving light emitted from the irradiation area or from an area on a backside of the irradiation area of the sheet. The line sensors are placed in the lengthwise direction as well as in a direction perpendicular to the lengthwise direction so that the photoelectric conversion devices of at least one of the line sensors can receive the light. The scanning means causes either the combination of the linear light source and the detection means or the sheet to have movement relative to the other in a direction different from the lengthwise direction. The reading means obtains the initial image data by sequentially reading outputs from the photoelectric conversion devices of the detection means in accordance with the movement. The first conversion process carried out by the integration processing means is a process that generates pixel data corresponding to pixels divided in the lengthwise direction in the case where only one of the outputs in the initial image data is available for an area in the irradiation area, and to generate the pixel data corresponding to the pixels divided in the lengthwise direction by adding two or more of the outputs in the case where two or more of the outputs are available for an area in the irradiation area. The second conversion process carried out by the integration processing means is a process that generates data of a final image by adding the pixel data over a predetermined number of pixels consecutively lined in the lengthwise direction.
As the linear light source, a fluorescent lamp, a cold-cathode fluorescent lamp, an LED array or the like can be used. The linear light source itself may have a linear shape, as in the case of a fluorescent lamp or the like. Alternatively, the linear light source may cause the excitation light to be emitted linearly, and a broad area laser and the like can also be used. The excitation light emitted from the linear light source may be emitted continuously. Alternatively, the excitation light may be emitted continually as in the case of pulses generated by repeated emission and stoppage. In terms of noise reduction, a high-power pulsed light is preferred.
The direction in which the combination of the linear light source and the line sensors is moved relative to the sheet (the direction different from a lengthwise direction of the combination) is preferably a direction substantially perpendicular to the lengthwise direction of the combination, that is, a direction along a short axis thereof. However, the movement direction is not limited to the short-axis direction. For example, the movement direction may be oblique to the direction substantially perpendicular to the lengthwise direction of the linear light source and the line sensors as long as the excitation light can be irradiated substantially over the entire sheet. Alternatively, the movement may form a zigzag pattern.
The linear light source and the line sensors maybe located on the same side of the stimulable phosphor sheet or located separately on different sides of the sheet. However, in the case where the linear light source is placed on the side different from the side of the line sensors, a base material or the like of the stimulable phosphor sheet needs to be transparent to the light so that the light can reach the side opposite the side of excitation light irradiation.
As the photoelectric conversion devices comprising the line sensors, amorphous silicon sensors, CCD sensors, CCDs with back illuminators, and MOS image sensors can be used, for example.
As the stimulable phosphor sheet for recording the radiation image information, an ordinary stimulable phosphor sheet serving as phosphor for absorbing the radiation and as phosphor for storing the radiation energy (i.e. for recording the radiation image information) will suffice. However, a stimulable phosphor sheet proposed in Japanese Patent Application No. 11(1999)-372978 may be preferably included as the sheet to be read by the radiation image information reading apparatus of the present invention. This sheet separates the function of absorbing the radiation from the function of storing the radiation energy, unlike the conventional stimulable phosphor sheet. In this sheet, phosphor having excellent radiation absorption and phosphor having excellent responsiveness to light emitted by the absorption are used separately for absorbing the radiation and for recording the radiation image information. The phosphor having excellent radiation absorption (radiation absorption phosphor) is used for absorbing the radiation and causes ultraviolet to visible light to be emitted therefrom, and this light is absorbed by the phosphor having the excellent responsiveness to this light (recording phosphor) to store the energy thereof. The energy is released as the light upon exposure to the excitation light such as visible to infrared light, and the light emitted in this manner is photoelectrically read by photoelectric reading means. In this manner, radiation image information detection efficiency, that is, a radiation absorption ratio, a light emission efficiency, a light output efficiency, and the like can be improved. Therefore, it is preferable for the stimulable phosphor sheet used in the radiation image information reading apparatus of the present invention to include the recording phosphor.
The recording phosphor absorbs the ultraviolet to visible light emitted from the radiation absorption phosphor, and stores the energy as the radiation image information. Since the ultraviolet to visible light is emitted by absorption of the radiation by the radiation absorption phosphor, the radiation image information includes the image information recorded in the recording phosphor.
The term xe2x80x9cinitial image dataxe2x80x9d refers to signal data output from the photoelectric conversion devices and not subjected to the first conversion process and the second conversion process.
As a manner of arranging the line sensors comprising the detection means, it is preferable for the line sensors to be lined up in a main scan direction (a direction shown by an arrow X in FIG. 9A) without a gap between any two of the line sensors adjacent to each other. However, as shown in FIG. 9B, the line sensors may be arranged at intervals. In other words, any arrangement enabling the light from the irradiation area of the sheet to be received by a light reception area (an area in which the photoelectric conversion devices are located) of at least one of the line sensors can be used. In FIG. 9, the line sensors are arranged consecutively in a direction perpendicular to the main scan direction (a direction shown by an arrow Y) without a gap. However, the line sensors may be arranged separately, sandwiching the irradiation area.
In the present invention, the line sensors are lined in the main scan direction and the direction perpendicular to the main scan direction to obtain the initial image data by reading the outputs from the photoelectric conversion devices of the line sensors. The initial image data are converted (subjected to the first conversion process) into the pixel data for pixels divided in the main scan direction (whose size is smaller than a pixel size of the final image). The predetermined number of the pixel data are added (subjected to pixel density conversion) to obtain the pixel data of the final image (the second conversion process).
The first conversion process carried out on the initial image data comprising the outputs from the photoelectric conversion devices of the line sensors arranged in the main scan direction and in the direction perpendicular to the main scan direction is a process that converts the initial image data into the pixel data for one line of pixels in the main scan direction corresponding to a target area of initial image data reading, that is, to generate the pixel data corresponding to the irradiation area of the sheet. More specifically, in the case where only one of the outputs in the initial image data is available for an area in the irradiation area (that is, in the case where the light emitted from the area in the irradiation area is received by the photoelectric conversion devices of only one of the line sensors), the output is used to generate the pixel data corresponding to pixels divided in the main scan direction. However, in the case where the two or more of the outputs are available for an area in the irradiation area (that is, in the case where the light emitted from the area in the irradiation area is received by the photoelectric conversion devices of two or more of the line sensors), the outputs are added and converted into the pixel data corresponding to pixels divided in the main scan direction. For example, if a line sensor A, a line sensor B and a line sensor C are arranged as shown in FIG. 10, outputs from photoelectric conversion devices a1, a2, a3, . . . , b1, b2, b3, . . . , and c1, c2, c3, . . . of the line sensors are detected for an irradiation area of a stimulable phosphor sheet. The outputs comprise initial image data. For each of the areas in the irradiation area corresponding to the photoelectric conversion devices a1, a2, a3, and a4, the number of outputs is one. Likewise, for a5, a6, c5, c6, c7 and c8, the number of the outputs is also one due to insensitive areas (Wa, Wb and Wc) of the respective line sensors. Meanwhile, for each of areas in the irradiation area corresponding to the photoelectric conversion devices a7, a8, a9 and a10, the number of the outputs is not one. The outputs from the photoelectric conversion devices c1, c2, c3 and c4 are also available, which is the same for areas corresponding to the photoelectric conversion devices b1, b2, b3 and b4. In the first conversion process of the present invention, the outputs from the photoelectric conversion devices a1 to a6 and c5 to c8 are converted into pixel data corresponding to pixels divided in a main scan direction (a direction shown by an arrow X), and the outputs from pairs a7 and c1, a8 and c2, a9 and c3, a10 and c4, b1 and c9, b2 and c10, b3 and c11, and b4 and c12 are respectively added for each of the pairs to be used as the pixel data for each of pixels corresponding to positions of the respective pairs. As the addition process, simple addition, averaging, weighted operation, mask operation and the like can be used. When each of the outputs from only one of the photoelectric conversion devices is converted into the pixel data corresponding to each pixel, this conversion needs to be carried out in accordance with the addition process. In other words, if the addition process is simple addition for the case shown in FIG. 10, each of the outputs from the photoelectric conversion devices a1 to a6 and c5 to c8 is doubled to be used as the pixel data corresponding to each pixel. However, if the addition process is an averaging process, each of the outputs can be used as it is as the pixel data corresponding to each pixel.
The xe2x80x9csecond conversion processxe2x80x9d refers to a process that generates the data for the final image by adding the pixel data having been subjected to the first conversion process for a predetermined number of pixels. This process may be simple addition or averaging. If necessary, the process can be a weighted operation or a mask operation. The xe2x80x9cfinal imagexe2x80x9d refers to an image comprising the data that have been subjected to the first conversion process and the second conversion process. The final image comprising the digital data is provided to image processing means or image display means or image recording means, for example.
The second conversion means may be carried out after the first conversion process or concurrently with the first conversion process.
The radiation image information reading apparatus of the present invention preferably has an equalization processing means for correcting unevenness in the line sensors, the photoelectric conversion devices, and the excitation optical system and the reading optical system by carrying out an equalization process on the image data after the first conversion process and the second conversion process, in order to obtain a higher quality final image.
In order to improve image quality, it is preferable for the equalization means to carry out at least two processes from among the following: dark current correction processing (processing for correcting the image data so as to eliminate an effect of signals output from the photoelectric conversion devices when no light enters the devices), sensitivity correction processing (processing for correcting uneven sensitivity of the photoelectric conversion devices), linearity correction processing, and shading correction processing for correcting shading affected by unevenness in the excitation light or in the reading optical system.
According to the radiation image information reading apparatus of the present invention, the line sensors are arranged in the main scan direction and in the direction perpendicular to the main scan direction so that the light emitted from the irradiation area of the sheet is received by the photoelectric conversion devices in at least one of the line sensors. The initial image data comprising the outputs from the photoelectric conversion devices in the line sensors are subjected to the first conversion process to generate the pixel data corresponding to the pixels divided in the main scan direction by carrying out the addition process on two or more of the outputs from the same area if the two or more of the outputs are available for the area. The pixel data corresponding to respective pixels are then subjected to the pixel density conversion process (the second conversion process) to generate the data of the final image. Therefore, problems such as undetectable areas caused by insensitive areas of the line sensors are solved while a dynamic range of the apparatus is widened. In this manner, image quality degradation such as unevenness in the final image after the pixel density conversion process can be prevented.
The second conversion process may be carried out after or concurrently with the first conversion processing on the initial image data.
If the equalization means for carrying out the equalization processing on the image data after the first conversion process and the second conversion process is used for the radiation image information reading apparatus, the quality of the final image can be improved. Especially, the quality can be improved with certainty by carrying out at least two processes from among the following: dark current correction processing, sensitivity correction processing, linearity correction processing and shading correction processing.
The equalization process is carried out on the data after the first conversion process and the second conversion process which have comparatively fewer pixels and thus a smaller data amount. Therefore, memory space for data storage can be saved and an operation load for the equalization process can be reduced.